The invention generally relates to a gas sensing system and method, and more particularly to a hollow-core waveguide-based Raman system and method.
Homonuclear diatomic molecules are generally difficult to detect and measure. Such molecules as nitrogen and hydrogen, for example, do not absorb light under standard pressure and temperature conditions. They are, therefore, difficult if not impossible to detect and quantify with optical absorption based techniques. Further, oxygen has a weak forbidden absorption band that is difficult to use for reliable quantitative measurements. Most common analytical methods are based on low temperature gas chromatography.
There are few reliable techniques known for high accuracy and precision detection and quantification of such molecules. Due to the symmetry of such molecules, they are Raman active, making it possible to identify these molecules with high selectivity based on their Raman spectral peaks.
Raman sensing is widely applied for detection of various chemical compounds and biomaterials. Raman spectroscopy measures the frequency change and intensity of inelastically scattered light from interaction between molecules and monochromatic light. The spectral shift of Raman scattering can be associated with the interaction of an incoming photon and the molecule. The photon loses or gains energy interacting with specific vibrational, rotational, or electronic energy states of the molecule. It is therefore possible to identify molecules from their Raman peak positions, which indicate various molecular energy levels. Raman scattering is a function of the incident light power (I0), the inverse wavelength (λ) of the incident light to the 4th power, the concentration of material in the beam (c), and the scattering cross section of the molecule (J). In addition, any experimental setup and/or sample has its own restrictions on the ability of the instrument to collect and analyze light. This factor is usually called the instrument factor (K). Therefore, a simplified equation for the observed Raman Scatter can be expressed as follows:R=I0cJK/λ4  Eqn. (1)Clearly, if the experimental conditions are controlled, Raman spectroscopy should be capable of quantitative analysis, i.e., the intensity of a Raman signal is proportional to the partial pressure or concentration of the molecule. The Raman cross section is multiplied by a concentration factor. For many Raman samples, the factor is essentially one for a solid or liquid. For a gas sample, the factor is approximately 4.5×10−5. Therefore, Raman sensing of low-density media such as gases is very challenging. Strong laser power around watt level and high gas pressure from 50 to 100 atmospheres are usually employed. Also, quantification of the concentrations of gas components in a mixture requires strong signals, especially at low concentrations. Therefore, enhancing techniques are required to produce and collect Raman scattered photons from a mixture of gases.
Raman signals are intrinsically weak, roughly ten to sixteen orders of magnitude smaller than fluorescence. To achieve lower detection limits, surface enhanced Raman (SERS) and/or resonance Raman have been used to improve the Raman signal of certain chemicals. SERS requires absorbing the target molecules onto a roughened metal surface. Resonance Raman requires strong coupling between vibrational and electronic levels; therefore, they are not universal.
One known method for increasing the intensity of a Raman signal is Coherent anti-Stokes Raman Spectroscopy (CARS), which is a nonlinear optical method using two or more intense beams of light to generate anti-Stokes blue-shifted Raman signals. CARS experiments are not routine and are strongly dependent on the reproducibility of the performance of expensive lasers. See, for example, Begley, R. F., et al., “Coherent anti-Stokes raman spectroscopy, Applied Physics Letters, 25, 387 (1974). Much emphasis has therefore been placed on improving the interactions between light and gas analytes, which usually involves a multi-pass arrangement where the illumination laser beam is focused on the sample volume from a variety of directions. Gains of 10 to 100's are about all that is possible from this approach. This approach, however, has its limitation in that optical mirrors are susceptible to contamination. Loss of power is inversely proportional to reflectivity to the power of the number of reflections. Even for moderately efficient cells, the number of reflections can be between 25 and 100 therefore even very mild contamination can have devastating effects on cell efficiency.
Photonic bandgap fibers are known and commercial products in certain ranges are available. See www.crystal-fibre.com. These fibers employ a central hollow core surrounded by a honeycomb structure. Contrary to traditional fiber optics that relies on refractive index difference to guide light, photonic band gap fibers guide light based on the band gap created by periodic structure of air holes. More than 95% of the light is guided through the central core. See, G. Humbert, J. C. Knight, G. Bouwmans, P. S. J. Russell, D. P. Williams, P. J. Roberts, and B. J. Mangan, “Hollow core photonic crystal fibers for beam delivery,” Opt. Express 12, 1477-1484 (2004); Russell, P. “Photonic crystal fibers”, Science, 299, 358-262. 2003. The dimensions of both the core and honeycomb can be customized to yield fiber specifically tuned to a particular wavelength. Guiding light in the hollow core holds many applications that were not possible before. It has been used for IR absorption measurement of weak absorbing gases. See, T. Ritari, J. Tuominen, H. Ludvigsen, J. Petersen, T. Sørensen, T. Hansen, and H. Simonsen “Gas sensing using air-guiding photonic bandgap fibers”, Optics Express, Vol. 12, Issue 17, pp. 4080-4087. Particularly for Raman spectroscopy, the hollow core provides long interaction lengths between gas and laser while keeping the laser beam tightly confined in a single mode. The photon intensity inside the hollow core is very large due to the micron-size space. This has the potential of greatly enhancing the gas phase spectrum of nitrogen or any other contained gas. For example, see U.S. patent publication 2006/0193583. Further, the use of photonic bandgaps in a Raman device is also known. See, for example, U.S. Pat. No. 7,283,712 (hereinafter, “the Shaw patent”). The Shaw patent discloses a gas filled hollow core chalcogenide photonic bandgap fiber Raman device. The specific Raman device of the Shaw patent is designed for infrared light. Further, the specific Raman device of the Shaw patent includes a doped portion.
One homonuclear diatomic molecule, nitrogen, is a critical component found in natural gas. The development of an approach that would enable direct measurement of nitrogen would be critical in developing an inferential energy meter for the natural gas industry. It would therefore be advantageous to develop a new approach to detecting and quantifying homonuclear diatomic molecules.